Rheology concerns the deformation and flow of matter. Materials can resist deformation in a solid-like manner or in a viscous-like manner. Solid-like materials store energy under deformation, upon removal of the stress, the material returns to its undeformed state. Viscous-like materials dissipate stress during deformation. Materials with combined solid-like and viscous-like properties are said to be viscoelastic.
Most materials such as foodstuffs, paints, gels and polymer melts are viscoelastic in nature. Their viscoelasticity is often essential to performance criterion and consequently oscillatory strain techniques have been developed for measuring these properties. The solid-like or viscous-like nature is quantified from the stress response to an oscillatory strain. In ideal solid-like materials, the stress response will be in phase with the oscillatory strain, ie the largest deformation will coincide with the largest stress. Viscous-like materials do not store energy and the stress developed depends on the rate of deformation. Consequently, the stress response will be out of phase with the oscillatory strain. In viscoelastic materials, the stress will be between 0° and 90° out of phase with the deformation, displaying viscous-like and solid-like properties. Storage modulus is defined as a measure of the solid-like or in-phase stress response. The loss modulus measures the fluid-like response.
Rheological, particuarly viscosity, measurements are important for process control. Flow properties often dictate the performance of processing operations. Highly viscous products may be undesirable due to expensive pumping costs, however low viscosity products may be prone to settling problems. Flow properties may be critical to performance in the desired end use. For example, the surface roughness of painted films during and after application is controlled through tailoring very specific flow resistance in paint products. Furthermore, flow properties are dictated by the inherent material microstructures. Consequently, rheological measurements that gauge material conversion rates and addition levels during processing may predict many performance criterion after processing.